Polypropylene and other polyolefin resins are inexpensive and have excellent properties, such as mechanical physical properties and chemical properties. Hence, they are used in large amounts in a wide range of fields, including automobile etc. However, since polyolefin resins do not have a polar group in the molecule, they generally have a low polar surface, which problematically makes coating and bonding difficult.
For this reason, modified polyolefins are used for coating and bonding of polyolefin resins after being dissolved in organic solvents, such as toluene and xylene. However, making modified polyolefins water soluble is desirable in terms of environmental problems, safety-and-health problems, and the like, and many studies are being conducted on how to make modified polyolefins water soluble.
For example, PTL 1 discloses a method for obtaining an aqueous resin composition, the method comprising adding an amine to a toluene solution of acid-modified chlorinated polypropylene, adding a surfactant after mixing the mixture, gradually adding water for emulsification, and removing the toluene under reduced pressure. However, foaming is likely to occur in the step of removing the toluene due to the presence of the surfactant. This causes a problem of longer processing time because it is necessary to remove the toluene while suppressing foaming.
Moreover, PTL 2 discloses a method for obtaining an aqueous resin dispersion, the method comprising melting a modified polyolefin, mixing a surfactant and/or alcohol and amine as an auxiliary agent for making the modified polyolefin water soluble, and adding water at a high temperature and high pressure. However, since the modified polyolefin is made water soluble by melting, it is difficult to make the modified polyolefin water soluble if the melt viscosity is too high when the modified polyolefin is made water soluble. Thus, the molecular weight of usable modified polyolefin is limited. Furthermore, since it is substantially necessary to add a large amount of auxiliary agent for making the modified polyolefin water soluble, there are problems such that it is necessary to reduce the water resistance of the dry film of the aqueous resin dispersion, and that the device used for making the modified polyolefin water soluble should be a reactor having resistance under high temperature and high pressure.
In order to improve these problems, methods for producing an aqueous resin composition by reacting a modified polyolefin with a surfactant are being investigated. For example, PTL 3 and PTL 4 disclose production methods wherein an acid modified polyolefin is reacted with a hydrophilic polymer, and dissolved in an organic solvent, followed by addition of water for dispersion. However, since a large amount of hydrophilic polymer is substantially used in the reaction, there are problems such that the physical properties of the coating film as a coating agent are adversely affected; for example, the water resistance of the dry film of the aqueous resin dispersion is reduced.
In order to improve these problems, methods for producing an aqueous resin composition without using an auxiliary agent for making modified polyolefins water soluble are being investigated. For example, PTL 5 discloses a production method using a specific type of organic solvent.